1. Field of Invention
The invention relates to a method and a device for conditioning a periodic analog signal to predetermined positive and negative desired peak values.
2. Description of the Related Art
In position measurements, and in particular measuring distances, rotational speeds or angles of rotation, use is made of incremental measurement systems using sensors. Sensors provide two alternating periodic electrical signals being phase-displaced by 90° relative to each other. Signals of this type are also called orthogonal. The sensor usually senses a material measured, e.g. a marked code disk or a code ruler. The generated signals are in most cases of sinusoidal shape, but may also be triangular or trapezoidal in shape. For the sake of simplification, reference will be made in the following to sine and cosine signals. The amplitudes of sinusoidal signals in sensors or other devices vary, are age-dependent, depend on operating frequency, temperature, and supply voltage, and are subject to the influence of the material measured. These influences may lead to fluctuations in signal deviation, including fluctuations of signal amplitude. However, fluctuations in signal offset are also possible. Finally, signal phase deviations may also occur, in particular due to geometric or mechanical shift or changes in the sensor in connection with the scanned material measure.
In order to achieve synchronization in such incremental measurement systems, a synchronization signal is also provided in many cases which, due to its asymmetrical, periodic signal shape, enables a synchronization of the alternating signals to the material measure and thus a unique definition of a coordinate system.
Faulty electrical signals considerably limit the performance of such measurement systems and lead to errors in the detected distances, rotational speeds or angles of rotation.
A method for correcting the aforementioned fluctuations of amplitude is known from patent DE 38 43 108. The known method includes checking whether the positive and negative peak values of the sine and cosine signals have their peaks within a certain window. If this is not the case, the signal is corrected thereafter by suitable amplifications or by suitable additions. The type of correction is selected according to whether the signal is greater or less than the window, or to which correction was performed in the previous half-wave. Therefore, signal deviations from the window may require many periods for necessary conditioning. This may have an adverse effect on the first calibration upon power-on or even make necessary signal conditioning virtually impossible.
Patent EP 0,489,936 proposes a further improvement to the method described in patent DE 38 43 108, which reduces the circuitry overhead.
Patent application DE 197 12 622 describes an arrangement and a method for correcting erroneous signals in incremental measurement devices. For signal conditioning, a microprocessor is employed, which determines correcting factors in a closed loop in order to achieve higher precision.
The document “Die neue Dimension in der Weg- and Winkelmesstechnik”, R. Burgschat, F & M, Feintechnik, Microtechnik, Microelektronik, Volume 10, 1996, discloses the use of digital potentiometers in an application specific integrated circuit (ASIC) for signal correction.
It is known from patent application DE 42 42 145 to correct a phase deviation of sinusoidal signals by summing and subtracting both signals to obtain further sinusoidal signals which no longer exhibit the statistic phase deviation, but have an exact phase shift of 90°.
Based upon the foregoing, there is a need for a technique which simply and effectively conditions periodic signals within a relatively short period of time.